add doc comments to std.fs.File.default_mode

lib/std/fs/file.zig

@@ -48,6 +48,12 @@ pub const File = struct {
         Unknown,
     };
 
+    /// This is the default mode given to POSIX operating systems for creating
+    /// files. `0o666` is "-rw-rw-rw-" which is counter-intuitive at first,
+    /// since most people would expect "-rw-r--r--", for example, when using
+    /// the `touch` command, which would correspond to `0o644`. However, POSIX
+    /// libc implementations use `0o666` inside `fopen` and then rely on the
+    /// process-scoped "umask" setting to adjust this number for file creation.
     pub const default_mode = switch (builtin.os.tag) {
         .windows => 0,
         .wasi => 0,